In a digital transmission system, a multilevel modulation scheme is often used to allow transmission of a larger amount of information in the frequency bandwidth that is usable for each service. In order to increase the frequency utilization efficiency, it is effective to increase the number of bits allocated per symbol of the modulation signal (modulation order), but the relationship between the upper limit on the rate of transferrable information at a frequency of 1 Hz and the signal-to-noise ratio is limited by the Shannon limit. Digital satellite broadcasting is one example of a form of information transfer using a satellite channel.
In digital satellite broadcasting, a Travelling-Wave Tube Amplifier (TWTA) with a good power efficiency is often used due to limitations on the hardware of the satellite transponder. For maximum utilization of the limited hardware in the satellite transponder, the amplifier is preferably caused to operate in the saturation region, so as to maximize the output of the satellite transponder. Distortion generated in the amplifier, however, leads to transmission degradation. Therefore, Phase Shift Keying (PSK) is often used as a modulation scheme having strong resistance to transmission degradation caused by distortion generated in the power amplifier. Currently, a transmission system called ISDB-S is used in Japan as a transmission system for digital satellite broadcasting, and PSK modulation such as BPSK, QPSK, or 8PSK can be used.
Amplitude Phase Shift Keying (APSK) can be used in DVB-S2, which is a European transmission system, and a modulation scheme that further improves the frequency utilization efficiency has been put into practical use. For example, transmission having a maximum of 4 bps/Hz as the frequency utilization efficiency is possible with 16APSK, and transmission having a maximum of 5 bps/Hz is possible with 32APSK.
In digital satellite broadcasting currently in use, information is corrected in a transmission device that uses an error-correcting code. By adding a redundant signal called parity bits to information that is to be transmitted, the redundancy (code rate) of the signal can be controlled, allowing the resistance to noise to be increased. Error-correcting codes and modulation schemes are closely related, and the theoretical upper limit on the frequency utilization efficiency with respect to the signal-to-noise ratio is called the Shannon limit. Low Density Parity Check (LDPC) codes were proposed by Gallager in 1962 as strong error-correcting codes that have the property of approaching the Shannon limit (for example, see R. G Gallager, “Low Density Parity Check Codes” (NPL 1)).
An LDPC code is a linear code defined by an extremely sparse check matrix H (a check matrix with entries of 0 and 1, in which the number of 1's is extremely small).
The LDPC code is a strong error-correcting code for which transmission characteristics approaching the Shannon limit are obtained by increasing the code length and using an appropriate check matrix. An LDPC code is used in DVB-S2, in the transmission system for advanced wide band digital satellite broadcasting described in ARIB STD-B44 (referred to below as the advanced satellite broadcasting system; for example, see the Transmission System for Advanced Wide Band Digital Satellite Broadcasting, ARIB Standard ARIB STD-B44 Version 1.0 (NPL 2)) and also in the IEEE802.16e Standard for Broadband Wireless Access. By combining multilevel APSK modulation with a strong error-correcting code, a representative example of which is an LDPC code, transmission at a higher frequency utilization efficiency is becoming possible.
Taking the advanced satellite broadcasting system as an example, the code length of the LDPC code in this system is 44880 bits, and the code is configured with a Forward Error Correction (FEC) frame. This code has been shown to exhibit performance within approximately 1 dB of the BPSK limit (the theoretical upper limit on the frequency utilization efficiency with respect to the signal-to-noise ratio when using BPSK in the constellation diagram) (for example, see Suzuki et al., “Design of LDPC codes for the Advanced Satellite Broadcasting System” (NPL 3)).
In the advanced satellite broadcasting system, 41/120 (≈1/3), 49/120 (≈2/5), 61/120 (≈1/2), 73/120 (≈3/5), 81/120 (≈2/3), 89/120 (≈3/4), 97/120 (≈4/5), 101/120 (≈5/6), 105/120 (≈7/8), and 109/120 (≈9/10) are established as the LDPC code rates (for example, see NPL 2). Apart from these, an LDPC code rate of 11/40 has been proposed (for example, see JP 4688841 B2 (PTL 1) and JP 4856608 B2 (PTL 2)).